laser cutter | George Smart

While tinkering with a homebrew GPSDO project, I spent a bit of time searching the depths of the internet for information and parts for my project. I found a PCB for a Symmetricom GPSDO (specifically the Symmetricom 089-03861-02 as per the PCB silkscreen in my case, although the firmware reports itself as 090-03861-03). The board was cheap because the OCXO was missing – I suspect it had aged beyond where the error voltage range was specified, making it unusable. But the board was around £15 GBP delivered from AliExpress, so I took a chance on it based on the fact it had a Furuno GT-8031F GPS receiver which is a GPS module specifically designed for timing applications which “delivers highly accurate GPS timing”. The module also had a CPU (Renesas F2317VTE25V-H8S/2317) with accompanying flash and RAM ICs as well as a Xilinx Spartan-3 FPGA (XC3S200).

Besides the missing OCXO module, my board worked perfectly. I was able to piece most of the information together from the following two resources:

I saw that some people had put their units into Hammond Manufacturing project boxes, and that gave me a few ideas. It would be nice to box the unit up with some ancillary electronics to share the UART (57600 baud, 8N1) over Ethernet TCP/IP. However, my experience of UART to Ethernet modules has always been poor and friends reported similar, so I opted for an FTDI-based USB interface (namely the FT232RL, as the cheapest/easiest part during the chip shortage of 2019-2022). I was tempted to use a Maxim MAX232 device to perform the necessary conversions, but, in the end opted for two NPN transistors – a bit hacky, but much cheaper. I may well come back to the Ethernet option in the future.

The carrier takes 5V at around 2A input on a 2.1mm x 5.5mm DC power socket, a USB-B connector and holds five LEDs: four main LEDs from the GPSDO (which are connected to test-points on the PCB, I can’t find the LED signals on any connector), plus one LED from the FTDI device showing USB activity.

The image below shows where the LED signals are taken from. For reference, the big IC in the centre is the Xilinx Spartan FPGA:

The LED signals are all common anode, fed from +3.3V taken from the board. The LEDs are fed through a resistor, and the individual cathodes connect back to either the CPU or FPGA through another of the wires:

RED: +3.3V supply used to power the LED anodes
BROWN: Alarm
WHITE: Activity
YELLOW: Heartbeat (DS2)
ORANGE: Error (DS1)

There are two other LED signals (DS3 and DS4) which aren’t connected.

From here, I created a carrier board which had the correct mounting holes for the Symmetricom module, and offered front LEDs to show status, an easy 5V interface and a USB-UART interface to the module control port. The final board looks as below:

With the module fitted, the board looks more like the following. Note, this was an earlier version of the PCB:

The project was designed to fit inside a Hammond Manufacturing 1455N1601BK case which takes a 160mm long by 100mm wide PCB, and the designed PCB fits the case nicely. By default, the Hammond case comes with either aluminum or plastic end plates, but I went to the effort to make PCB front and rear plates to enclose the case. Using PCB meant I was able to use tools I was familiar with to create the end places, and use the same order as the main carrier PCB. The copper on the PCB could be used to create a metal Faraday screen to enclose any electrical switching noise, and the silkscreen could be use to add legends, logos and decals.

I prototyped the designs as below in the PCB package and then used my laser cutter to test them out for fit. A test fit of the front panel is shown below:

The below picture shows the PCB end boards as they arrived and the cardboard cutouts. There are some small tweaks, between the two, but overall the process worked well. One thing to note is that the internal cutouts for the BNC connectors are a little bit tight – in future I need to add an extra 0.5mm or so clearance (in addition to the 0.5mm clearance I left already). However, the connectors pushed in even if a little tightly.

Final assembly went well, and I am very pleased with the results:

While looking for a way to create front panels and detailing on homebrew equipment, I was pointed to a YouTube video by Mark Presling entitled Metal Finishing With Mark – Metal Engraving 101.

In the video, Mark explains how cold galvanizing zinc spray, when ‘excited’ by a laser, burns at a high temperature to permanently mark the surface of the material onto which the zinc was sprayed. Mark suggests that this only works on stainless steel, however, other videos show how it can be used on ceramics, glass and similar substrates to burn or melt the substrate. I’m not exactly sure of the process, but, it certainly does leave a controllable, visible mark on the surface, which is exactly what I was after!

The box above shows markings for the 144 MHz antenna, GPS antenna, and status LEDs for an APRS transmitter I happened to be working on at the time. The effect is to leave a darker surface on the Hammond diecast box, which (at least to my testing) is very hard wearing and does not come off with use of solvents…

Here’s how

Firstly you’ll need to coat the surface to be etched with a liberal spray of zinc cold galvanizing compound. I used MOTIP Zinc Spray because it was the cheapest I could find on eBay and it works just fine – perhaps I got lucky but I’ve seen several videos on YouTube each swearing by a different make of spray, and they all appear to work. The important thing is that it is high in zinc. It’s an epoxy based aerosol, so, spray outside using the appropriate precautions. The spray should be quite thick, I spray on about 4 heavy coats one over the other and then let it ‘dry’ for around 5 minutes, just until the main solvent has evaporated.

While the spray is drying, design your artwork. I’m making a line drawing of the car along with my callsign to put on the box, mainly to see how it comes out – I’m keen to see if the line drawing comes out well or not – so watch this space! My design looks like the following:

Next we get to put the metal into the laser cutter. I use a 60W CO2 laser cutter, with the power set to around 50%. Others have reported success using 10W diode lasers. I found that 50% was about right for my machine. Going slowly helped a lot, I reduced the machine to around 5mm/second. Where possible, vector engrave as the laser power is continuous and more controlled than raster scanning, but for large areas, such as the text, raster scanning works fine. I always reinforce text with a vector engrave around the outer.

You’ll need to focus the machine as you’d normally do in order to cut the surface.

Once focused, frame the metal on the cutter bed. My laser cutter has a spotting laser which really helps with this.

At this point, you’re ready to go! When the paint is hit with the laser, it goes a very burnt/sooty black. The process generates some very nasty fumes, which you are well advised not to breath – this includes metal vapors which are incredibly dangerous.

Once the engraving is done, leave the work in the cutter’s fume extraction for a short while to be sure that the chamber is clear of toxics, and then remove the work. Mine looks like this:

The final stage in the process is to use a paint remover to remove the paint from the metal to reveal the final design. I use cellulose thinners, which works well. Be sure to do this in a well ventilated space, otherwise you end up with a headache (like I have now, as I write this!).

I think you’ll agree that the final result looks very clean and tidy, and has retained all of the detail present in the original design.

This process is quick and easy to do if you have a laser cutter, uses a cheap-ish (around £6) can of zinc spray, and produces good, repeatable results with minimal fuss. It’s very useful for creating front panels and similar.

You may also find that spraying another colour of paint over the top, and then sanding down very lightly will further accentuate the design.